Spinal Anaesthesia in Obstetrics

(See  Anaesthesia for Caesarean section, Update in Anaesthesia 1998;9:3)

There are several reasons for preferring spinal anaesthesia to general anaesthesia for Caesarean sections. Babies born to mothers having spinal (or epidural) anaesthesia may be more alert and less sedated, as they have not received any general anaesthetic agents through the placental circulation. As the mother's airway is not compromised, there is a reduced risk of aspiration of gastric contents causing chemical pneumonitis (Mendelson's syndrome).

Many mothers also welcome the opportunity of being awake during the delivery and being able to feed their child as soon as the operation is completed. There are, however, also disadvantages. It may be difficult to perform the spinal injection as the pregnant uterus will impede lumbar flexion and, if labour has started, the mother may be unable to remain still when having contractions. Unless small gauge needles (25 gauge) are used, the incidence of post-spinal headache may be unacceptably high. Spinal anaesthesia for Caesarean section should not be attempted until the anaesthetist has accumulated sufficient experience in non-pregnant patients.

In the absence of hypovolaemia due to bleeding, spinal anaesthesia is a simple and safe alternative to general anaesthesia for manual removal of a retained placenta. It does not produce uterine relaxation and if this is required, a general anaesthetic with a volatile agent may be preferred. 

Technique

Spinal anaesthesia is performed and managed in pregnant patients in the same way as in non-pregnant patients but with a number of special considerations.

• It is generally recommended that obstetric patients should be pre-loaded with not less than 1500mls of a crystalloid solution before the dural puncture is performed.
• Although spinal anaesthesia is not contra-indicated in the presence of mild pre-eclampsia, remember that such patients may have altered clotting function and are relatively hypovolaemic. There is always a chance that a pre-eclamptic patient may suddenly fit and anticonvulsant drugs (diazepam or thiopentone) must be immediately available. (See Update in Anaesthesia No. 9).
• Pregnant women need smaller volumes of spinal anaesthetic solution than non-pregnant women in order to obtain a given height of block. For a Caesarean section, anaesthesia should extend to T6 (about the bottom of the sternum) to be completely successful. This can usually be achieved with the following regimes, although the hyperbaric agents are more predictable:
  • 2.0-2.5ml of a hyperbaric solution of 0.5% bupivacaine or
  • 2.0-2.5ml of an isobaric solution of 0.5% bupivacaine or
  • 1.4-1.6ml of a hyperbaric solution of 5% lignocaine or
  • 2.0-2.5ml of an isobaric solution of 2% lignocaine with added adrenaline (0.2ml of 1:1000.

If anaesthesia is required for a forceps delivery, 1.0ml of a hyperbaric solution injected with the mother in the sitting position is usually adequate. Anaesthesia to T10 is needed for removal of a retained placenta. This can be obtained by injecting 1.5mls of a hyperbaric solution with the patient sitting and then lying her down. 

Positioning of the Pregnant Patient

Pregnant patients should never lie supine as the gravid uterus will compress the vena cava and, to a lesser extent the aorta (aorto-caval compression) resulting in hypotension. They should, instead, always lie with a lateral tilt. This can be achieved either by tilting the whole table or by inserting a wedge under the patients' right hip. The uterus is displaced slightly to the left and the vena cava is not compressed (See Update in Anaesthesia No. 9).

As with all patients undergoing surgery under spinal anaesthesia, oxygen should be given during the operation. As hypotension commonly occurs despite fluid pre-loading, many anaesthetists routinely give a dose of vasopressor intravenously. Ephedrine is the favoured vasopressor, as it does not cause constriction of the uterine blood vessels. If it is not available, one of the other vasopressors discussed previously should be used as untreated hypotension can seriously damage the unborn infant.

After delivery of the baby, syntocinon is the oxytocic of choice as it is less likely to produce maternal nausea and vomiting than ergometrine. 

Further reading:

• Collins C, Gurug A. Anaesthesia for Caesarean section. Update in Anaesthesia 1998;9:7-17
• Torr GJ, James MFM. The role of the anaesthetist in pre-eclampsia. Update in Anaesthesia 1998;9:17-22
• Morgan P. Spinal anaesthesia in obstetrics (a review). Canadian Journal of Anaesthesia. 1995;42:1145-63
• Ngan Kee WD. Intrathecal pethidine: pharmacology and clinical applications. Anaesthesia and Intensive Care 1998;26:137-146.
• Ramasamy D, Eadie R. Transient radicular irritiation after spinal anaesthesia with 2% lignocaine. British Journal of Anaesthesia 1996, 79, 394-395
• Rawal N, Van Zudert A, Holmstrom B, Crowhurst JA. Combined spinal epidural techniques. Regional Anaesthesia. 1997;5:406-423.
• Williams D. Subarachnoid saddle block using pethidine. Update in Anaesthesia 1998;9:47-8 

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